Bugfix: use b"" when joining in dev_rzusbstick.py

[martijnthe]

I ran into this exception when trying out zbwireshark (from killerbee-3.0.0b2) with an Atmel RZUSBSTICK. The problem was that bytes can't be join'd from a str.

Traceback (most recent call last):
  File "/home/linuxbrew/.linuxbrew/bin/zbwireshark", line 4, in <module>
    __import__('pkg_resources').run_script('killerbee==3.0.0b2', 'zbwireshark')
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/pkg_resources/__init__.py", line 651, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/pkg_resources/__init__.py", line 1448, in run_script
    exec(code, namespace, namespace)
  File "/home/linuxbrew/.linuxbrew/lib/python3.9/site-packages/killerbee-3.0.0b2-py3.9-linux-x86_64.egg/EGG-INFO/scripts/zbwireshark", line 108, in <module>
    main()
  File "/home/linuxbrew/.linuxbrew/lib/python3.9/site-packages/killerbee-3.0.0b2-py3.9-linux-x86_64.egg/EGG-INFO/scripts/zbwireshark", line 84, in main
    packet = kb.pnext()
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/killerbee-3.0.0b2-py3.9-linux-x86_64.egg/killerbee/__init__.py", line 442, in pnext
    return self.driver.pnext(timeout)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/killerbee-3.0.0b2-py3.9-linux-x86_64.egg/killerbee/dev_rzusbstick.py", line 608, in pnext
    ret[0] = ''.join(framedata[:-1])
TypeError: sequence item 0: expected str instance, bytes found

Fixes #239